1. Field of the Invention
The invention relates to thin-film solar cells and method of making.
2. Background Information
Photovoltaic (PV) cells are made of materials referred to as semiconductors, such as, silicon, which is currently the most commonly used. Basically, when light strikes the cell, a certain portion of it is absorbed within the semiconductor material. This means that the energy of the absorbed light is transferred to the semiconductor. The energy impacts the electrons, allowing them to flow freely. PV cells also all have one or more electric fields which act to force electrons freed by light absorption to flow in a certain direction. This flow of electrons is a current, and by placing metal contacts on the top and bottom of the PV cell, one can draw that current off to use externally. For example, the current can power a calculator. This current, together with the cell's voltage (which is a result of its built-in electric field or fields), defines the power that the solar cell can produce.
A display screen made with TFT (thin-film transistor) technology is a liquid crystal display (LCD), common in notebook and laptop computers, that has a transistor for each pixel (that is, for each of the tiny elements that control the illumination of your display). Having a transistor at each pixel means that the current that triggers pixel illumination can be smaller and therefore can be switched on and off more quickly. TFT technology is also known as active matrix display technology (and contrasts with “passive matrix” which does not have a transistor at each pixel). A TFT or active matrix display is more responsive to change. For example, when you move your mouse across the screen, a TFT display is fast enough to reflect the movement of the mouse cursor. (With a passive matrix display, the cursor temporarily disappears until the display can “catch up.”) Active matrix (also known as Thin Film Transistor or thin film transistor) is a technology used in the flat panel liquid crystal displays of notebook and laptop computers. Active matrix displays provide a more responsive image at a wider range of viewing angle than dual scan (passive matrix) displays.
In this context, an Si:H film is a film of silicon in which hydrogen is incorporated. The hydrogen content is approximately 3 to 20%.
Solar cells based on the semiconductor material silicon have been known for many years. These solar cells are usually produced from solid monocrystalline or polycrystalline silicon, typical thicknesses of a solar cell of this type being approximately 300 to 500 μm. These thicknesses are required firstly in order to ensure sufficient mechanical stability and secondly to achieve absorption of the incident sunlight which is as complete as possible. On account of the relatively large film thicknesses and the associated high consumption of material, and on account of the unavoidable need for a high-temperature step for doping of the silicon wafers (T≧1000° C.), solar cells of this type entail expensive manufacture.
As an alternative to these relatively thick silicon solar cells described above, in addition to the thin film solar cells based on amorphous Si:H (referred to below as a-Si:H), which have already been the subject of research for some 20 years, thin-film solar cells made from microcrystalline Si:H (referred to below as μc-Si:H) have in recent years become an established subject for investigation. This cell material is expected to have a similarly high efficiency to that of monocrystalline silicon, but to involve less expensive production processes, as are also known for a-Si:H. At any rate, the use of μc-Si:H is supposed to suppress the degradation in the efficiency under intensive illumination, which is inevitable when using a-Si:H. However, a number of significant points still currently stand in the way of commercial utilization of μc-Si:H as the functional layer in a thin-film solar cell. Unlike the solar cell using a-Si:H, which has a thickness of the photovoltaically active film of approximately 300 nm, the solar cell made from μc-Si:H, to achieve a similarly good utilization of the incident light, must be approximately 3000 nm thick, i.e. has to be thicker by a factor of 10. Therefore, an economic process must also allow the deposition rate of the microcrystalline material to be higher by this factor than that achieved for a-Si:H. An inexpensive substrate, preferably window glass or even standard plastics, appears to be indispensable as a further necessary feature for commercial utilization of the μc-Si:H. For this purpose, it is necessary to have available deposition methods which are compatible with the substrates, i.e. low-temperature processes (T<100° C. for plastic or T≦200 to 300° C. for glass which is provided with a transparent conductive film), and these processes must moreover still achieve high film-generation rates.
According to the prior art, microcrystalline silicon (μc-Si:H) can be applied in thin films to a support material at temperatures of greater than approximately 200° C. using various processes. For example, it can be deposited directly from the gas phase. By way of example, the following deposition methods are known: high-frequency glow discharge deposition (HF-PECVD), electron cyclotron resonance (ECR) process, electron cyclotron wave resonance (ECWR) process, sputter deposition, hot-wire (HW) technique, microwave CVD.
Furthermore, processes are also known in which μc-Si:H is produced by initially depositing a-Si:H from the gas phase, which is then transformed into μc-Si:H. The transformation of a-Si:H to μc-Si:H is known, for example, from the following documents.
For example, U.S. Pat. No. 5,470,619 describes the transformation of a-Si:H into μc-Si:H by means of heat treatment at a temperature of 450° C. to 600° C.
U.S. Pat. No. 5,486,237 describes a temperature-induced transformation of a-Si:H films into μc-Si:H films at 550° C. to 650° C. over a period of 3 to 20 hours.
U.S. Pat. No. 5,344,796 describes a process for producing a thin μc-Si:H film on a glass substrate. In this process, first of all a μc-Si:H film is generated on the substrate and serves as a seed layer, then a-Si:H is deposited on this seed layer by means of a CVD process. The a-Si:H is transformed into μc-Si:H by means of a heat treatment, preferably at between 580° C. and 600° C. for a period of from 20 to 50 hours.
U.S. Pat. No. 5,693,957 likewise describes the thermal transformation of a-Si:H films into μc-Si:H films at 600° C., the transformation of certain a-Si:H films into μc-Si:H being deliberately prevented by impurities formed by these a-Si:H films.
A microwave plasma CVD process for the production of a-Si:H and μc-Si:H films is described in U.S. Pat. No. 5,334,423, in which, in saturation mode, 100% of the microwave power is introduced.
Published International Application No. 93/13553 (corresponding to U.S. Pat. No. 5,231,048) describes a microwave CVD process for producing thin semiconductor films, the process pressure lying below the Paschen minimum. A microwave CVD process with controllable bias potential for the production of thin semiconductor films is described in document U.S. Pat. No. 5,204,272.
The production of μc-Si:H films by means of a microwave CVD process is described in U.S. Pat. No. 4,891,330, in which preferably at least 67% of hydrogen is added to the process or precursor gas in order to assist the formation of the μc-Si:H phase.
A plasma process for the production of a μc-Si:H layer is described in document Published International Application No. 97/24769 (corresponding to U.S. Pat. No. 6,309,906), the precursor gas being diluted with hydrogen and/or argon.
Furthermore, a plasma treatment of an a-Si:H film by means of an argon plasma is described in U.S. Pat. No. 4,762,803, and by means of a hydrogen plasma in Published International Application No. 93/10555 (corresponding to U.S. Pat. No. 5,387,542), in order to obtain a μc-Si:H film.
European Patent No. 0 571 632 A1 (corresponding to U.S. Pat. No. 5,387,542) has disclosed a plasma CVD process for producing a polycrystalline Si film on a substrate. For this purpose, firstly a thin, amorphous Si:H film is produced on the substrate by plasma-assisted CVD coating. Then, the amorphous Si:H film is subjected to a plasma-assisted treatment using a hydrogen plasma, the amorphous Si:H film being transformed into the ploycrystalline Si:H film.
Plasma-enhanced CVD coating in pulsed mode for the production of an amorphous Si:H film on a substrate is known from U.S. Pat. No. 5,618,758.
Furthermore, it is also possible to produce a μc-Si:H film by alternating deposition of a-Si:H films and subsequent treatment of this film using a hydrogen plasma. This process is generally referred to in the literature as the layer-by-layer (LBL) technique. The process by which the a-Si:H is transformed into μc-Si:H at atomic level has not to date been unambiguously explained (there are several models under discussion), but a competition process between the etching away of disadvantageous Si—Si bonds and hydrogen-induced restructuring of the network toward the crystalline phase, which is more favorable in energy terms, seems very likely.
Parameters which provide good a-Si:H films, i.e. those which are suitable for components, are often used for the deposition of the a-Si:H film. The thicknesses of the individual films which are reported in the literature typically lie between 1.4 nm and several 10 s of nm. On account of this relatively great variation in film thickness, the result is aftertreatment, or post-treatment, times using an H2 plasma which lie in the range from a few seconds to several minutes. The deposition processes used are HF-PECVD processes, in which, on account of the low excitation frequency, the deposition rates are relatively low.
HF-PECVD processes at most achieve maximum deposition rates (film thickness actually deposited divided by the time required for this deposition) which are significantly below 10 nm/min.
The following text provides literature references which represent the prior art of μc-Si:H deposition by means of the LBL technique:                Asano, A.; Appl. Phys. Lett. 56 (1990) 533;        Jin Jang; Sung Ok Koh; Tae Gon Kim; Sung Chul Kim, Appl. Phys. Lett. 60 (1992) 2874;        Otobe, M.; Oda, S.; Jpn. J. Appl. Phys. 31 (1992) 1948;        Kyu Chang Park, Sung Yi Kim; Min Park; Jung Mok Jun; Kyung Ha Lee; Jin Jang; Solar Energy Materials and Solar Cells, Vol. 34 (1994), 509;        Hapke, P.; Carius, R.; Finger, F.; Lambertz, A.; Vetterl, O.; Wagner H.; Material Research Society Symposium Proceedings, Vol. 452; (1997), 737.        
All the processes which have been used to date for the LBL technique give very low effective deposition rates (1–6 nm/min), which restrict commercial application. Furthermore, in the LBL processes which have been used to date, the individual film thicknesses (1 nm to a few 10 s of nm) cannot reliably be set with accuracy without a complex in situ measurement technique. This variation from the first step of the process is reflected in the second step. The result in particular is that the duration of the second step (H2 plasma treatment) cannot be determined with accuracy in advance. This means that the process is dependent on an inherent stability which cannot be achieved on an industrial scale.
Measures aimed at increasing the rate, for example by increased introduction of power (higher plasma densities) lead to an increase in the particle fraction in the film and therefore to a reduction in quality.
The literature and the abovementioned documents describe relatively high process temperatures (250–330° C.), which are evidently required in order to ensure sufficient film qualities (compact, i.e. dense films) and to ensure film adhesion. Therefore, thermolabile substrates cannot be coated.